As is known, a rotary fluid coupling enables liquid to be transferred between two relatively rotating parts, such as a stationary shaft and the hub of a shearing drum of a mining machine, the hub being rotatably mounted on the shaft. One such rotary fluid coupling is shown in copending application Ser. No. 135,404, filed Mar. 31, 1980. In that application, a rotary fluid coupling is provided wherein a bore in a hub element receives a shaft element which is rotatable relative thereto. An annular piston surrounds the shaft and is reciprocable within the bore. This piston forms the wall of a chamber which connects a duct in the shaft element to a duct in the hub element. High-viscosity lubricating material is disposed between the piston and a seal which surrounds the shaft element such that the seal, via the lubricant, is exposed to pressure exerted on the piston by fluid in the aforesaid chamber interconnecting the two ducts.
In a coupling of the type shown in the aforesaid application Ser. No. 135,404, the high pressure of the fluid in the chamber interconnecting the two ducts acts on the annular piston and applies to the lubricating material between the piston and the seal a pressure which corresponds to the pressure of the liquid flowing through the annular chamber. This precludes any transfer of fluid from one side of the annular piston to the other. The pressure acting on the piston causes only a small portion of the high-viscosity lubricant to issue through the gaps of the seal, preferably a labyrinth seal, and travel through bushings at opposite ends of the hub through which the aforesaid shaft extends.